1. Field of the Invention
The present invention relates to an objective lens system for a device for optically reading digitalized information on an optical disk such as DAD.
2. Description of the Prior Art
Conventional objective lens systems for use in a device for optically reading digitalized information need a collimator lens element (c) as shown in FIG. 6 when an objective lens is constituted by a single lens element or need an objective lens constituted by a plurality of lens elements as shown in FIG. 7 when no collimator lens element (c) is provided. In case of the former type, the collimator lens element (c) is always fixed while the objective lens element (L) is shifted, so that two lens-barrels are needed, resulting in difficulty of adjustment of the optical axis of the whole optical system.
As the objective lens systems of this type usually has an aspherical surface on the single objective lens element, optical performance is seriously deteriorated due to misalignment of the objective lens with the whole optical system and thus the adjustment of the optical axis of the single lens element to the whole optical system should be carried out very precisely. Also in case of the latter type, in which the objective lens element comprises a plurality of lens elements, it is necessary to arrange the lens barrel very precisely to maintain sufficient optical performance although an objective lens systems of this type need a larger space for the arrangement of the lens barrel as compared with those of the former type and distortion of the lens barrel due to the change of temperature causes misalignment of the lens elements, so that the optical performance tends to be deteriorated.